Processing of Si-based heterojunction
solar cells and properties of interfaces
A.G. Ulyashin, M. Scherff, M. Gao,
R. Sondermann, W.R. Fahrner
University of Hagen, Germany
Department of Electrical Engineering and
Information Technology (LGBE)
Outline of the Talk
• Introduction
• SANYO HIT Solar cells
• Experimental
• substrates, deposition of layers, metallization
•Experimental Results
• analysis by SRP, I-V, spectral response, XPS,
SIMS, spectroscopic ellipsometry, IR
termography, SEM, Raman spectroscopy
• Discussion
• Heterojunction or MIS solar cells?
• Summary
SANYO HIT SOLAR CELLS
Intrinsic a-Si:H layer
p-type
a-Si:H
n-type
a-Si:H
n-type Cz Si
ITO
Deposition conditions of a-Si:H films
Gas flow rate (sccm)
a-Si:H
SiH4
p-type
n-type
i-type
B2H6
(1%)
5-20
2-40
5-40
5-40
B2H6 , PH3:H2 balanced
PH3
(1%)
5-40
-
H2
Substrate Pressure RF power
temp. (°C) (Pa)
(mW/cm2)
20-100 80-180
5-200 80-180
0-100 80-180
10-40
10-40
5-40
Toru Sawada et al. in: First WCPEC, 1994, Hawaii, p.1219-1226.
5-90
10-60
5-90
Metal
PROCESS PARAMETERS
• N,P-type a-Si:H (µc-Si) layers deposition:
PECVD 13.56 / 110 MHz,
• ITO deposition:
Room temperature -230 °C, InSn, (InSn
90/10),ITOtargets
single magnetron in DC mode, 100
W/cm2 power density
• Front side metallization (e-gun):
Cr (30 nm) / Ag (3 µm)
• Back side metallization (e-gun):
Al (2 µm)
PROCESS SCHEME
• Substrate cleaning:
-rinzing in aceton (5 min,ultrasonic bath)
-rinzing in methanol(5 min,ultrasonic bath)
-oxide removal with HF (1min, 1%)
or RCA cleaning
• a-Si:H layer deposition
• ITO deposition
• Front grid deposition
• Masking of cells
• ITO etching (HF)
• A-Si:H layer etching by SF6 plasma
• Photoresist masking of front side
• Oxide removal on back side
• Al back contact evaporation
• Cleaning
E x te r n a l Q u a n t u m E f fi c i e n c y
1.0
0.8
0.6
5 nm
10 nm
0.4
15 nm
20 nm
0.2
30 nm
0.0
400
600
800
1000
Wavelenghth (nm)
1200
metal grid / ITO / (n)a-Si:H /(p)c-Si(multi-Si)/Al
solar cell parameters (1× 1 cm2)
__________________________________________________________________________________________
Si
Isc
Voc
FF
Eff
Substrate
(mA)
(mV) (%)
(%)
_________________________________________
Fz
31.6
587
78.4
14.6
Cz
30.9
588
79.3
14.4
Baysix
27.4
603
77.5
12.8
_________________________________________
+ Back surface field
_________________________________________
Cz
26.8
574
79.0
12.1
_________________________________________
+ (p)µc-Si (10nm) on the back side
_________________________________________
Cz
29.9
612
79.8
14.6
_________________________________________
(n)a-Si:H+(i) a-Si:H(20nm) on the front side
_________________________________________
Baysix
25.4
613
78.5
12.2
Resistivity (ohm.cm)
1x10
4
9x10
3
a-Si:H on Corning glass
8x10
3
7x10
3
6x10
3
0
50
100
150
200
Hydrogen flux (sccm)
Resistivity of a-Si:H layers deposited
on a Corning glass at 150 °C versus
hydrogen flux during the deposition.
12.5
Efficiency before etching
Efficiency after etching
Efficiency (%)
12.0
11.5
11.0
10.5
10.0
9.5
0
50
100
200
Flux of hydrogen (sccm)
Solar cells ITO(80nm) / (n)a-Si:H(5nm) / Cz
Si efficiencies versus hydrogen flux during
the deposition of a-Si:H layer (150 °C) before
and after the etching of edges.
Solar cell parameters
(~5 nm a-Si:H emitter deposited at different temperatures)
39 cm2 Solar Cells, Cz Si
Temperature
of a-Si:H
Jsc
deposition, °C (mA/cm2)
Uoc
(mV)
FF
(%)
η
(%)
230
31.3
586
68.5
12.6
150
30.4
590
70.2
12.6
100
30.2
568
66.3
11.4
RAMAN MEASUREMENTS
a-Si:H properties
140
120
Intensity (a.u.)
80
230 °C
150 °C
100
100 °C
60
40
20
0
-20
-40
1800
2000
2200
2400
2600
-1
Raman Shift (cm )
2 800
-4
6.0x10
ITO specific resistivity versus
deposition temperature
(ITO target, Ar ambient)
-4
-4
5.0x10
-4
4.5x10
-4
4.0x10
-4
3.5x10
-4
3.0x10
-4
2.5x10
0
50
100
150
200
Temperature
12.8
2
12.6
Efficiency, %
Resistivity (Ohmcm)
5.5x10
Efficiency of 39 cm Solar cells
versus ITO deposition temperature
12.4
12.2
12.0
11.8
11.6
150
200
250
300
Temperature, °C
350
ITO/a-Si:H interface
XPS
Intensity (a.u.)
SiO 2 /a-Si:H/c-Si
ITO/a-Si:H/c-Si
110
108
106
104
Si 2p
102
100
98
96
94
Binding Energy (eV)
ITO(2nm)/n-type a-Si:H(5nm)/c-Si structure
a-Si:H deposited at 230 °C
Formation of Si-O bonds on the ITO/a-Si:H interface
ITO/a-Si:H interface
XPS
3200
Intensity (a.u.)
3000
2800
2600
ITO/a-Si:H/SiO 2
In 3d 5/2
ITO/a-Si:H/c-Si
2400
In 3d 3/2
2200
2000
1800
1600
1400
1200
1000
800
600
400
200
0
460
455
450
445
440
Binding Energy (eV)
ITO(2nm)/n-type a-Si:H(5nm)/c-Si(SiO2) structures
a-Si:H deposited at 230 °C
Formation of Si-In bonds on the ITO/a-Si:H interface
Penetration of In in to the Si substrate
ITO/a-Si:H interface
XPS
Intensity (a.u.)
600
ITO/a-Si:H/SiO 2
550
ITO/a-Si:H/c-Si
Sn 3d 5/2
500
Sn 3d 3/2
450
400
350
300
250
200
500
495
490
485
480
Binding Energy (eV)
ITO(2nm)/n-type a-Si:H(5nm)/c-Si(SiO2) structures
a-Si:H deposited at 230 °C
Formation of Si-Sn bonds on the ITO/a-Si:H interface
Penetration of Sn in to the Si substrate
Intensity
ITO/a-Si:H interface
SIMS
10
5
10
4
In
10
3
Sn
10
2
10
1
10
0
10
-1
H
Si
0
5
10
15
20
M inutes
ITO(80nm)/n-type a-Si:H(5nm)/c-Si structure
a-Si:H deposited at 230 °C
Penetration of In, Sn, H in to the Si substrate
Look-in Thermography
Identification
Basic Material
Category
Area
η
: P543-9
: a-Si/c-Si hetero cell
: single junction solar cell
: 0,979 cm2
: 15,3 % (ISE Freiburg sertificated)
Bias Voltage
: 0,55 V
Current
: 4,55 mA
Temperature Scale
: 0 to 0,4 mK
Look-in Thermography
Identification
Basic Material
Category
Area
η
: P797
: a-Si/c-Si hetero cell
: single junction solar cell
: 39 cm2
: 0,15 %
Bias Voltage
: 0,2 V
Current
: 922 mA
Temperature Scale
: 0 to 2 mK
Look-in Thermography
Identification
Basic Material
Category
Area
η
: P846
: a-Si/c-Si hetero cell
: single junction solar cell
: 39 cm2
: 12,3 %
Bias Voltage
Current
Temperature Scale
: 0,55 V
: 98,6 mA
: 0 to 1 mK
SURFACE STRUCTURING BY HYDROGEN PLASMA
TREATMENT
80
Inte nsity (a.u .)
70
60
H 2 molecules in voids
50
40
30
20
10
0
3800
4000
4200
-1
Raman Shift (cm )
Raman spectra of H plasma
(110 MHz, 50 W, 400 mTorr, 200 sccm H2)
treated mc-Si at 250 °C for 5 min
SURFACE STRUCTURING BY HYDROGEN PLASMA
TREATMENT
SURFACE
0.5° BEVELED
SEM spectra of H plasma
(110 MHz, 50 W,400 mTorr, 200 sccm H2)
treated Si at 250 °C for 60 min
SUMMARY
- The surface preparation strongly influences the HJ solar cells properties.
A process in which first the complete front side of the solar cells is
processed is the optimum process for this kind of solar cells.
- Producing first the BSF results in low fill factors due to insufficient front
surface cleaning before the a-Si:H deposition on a substrate with Al on a
backside.
- A chemical etching procedure or passivation by silicon oxynitride of the
edge area of HJ solar cells leads to an improvement in the efficiency up to
~30%.
- Formation of Sn-Si, In-Si, Si-O bonds on the ITO/a-Si:H interface was
observed by XPS measurements which shows that during the ITO
deposition a penetration of Sn,In,O atoms into the Si substrate occurs.
This conclusion was supported also by results from SIMS measurements
and spectroscopic ellipsometry (for Si-O bonds formation).
- An increase of the ITO deposition temperature leads to the increase of the
conductivity of this material and in the same time to the decrease of the
solar cell efficiencies ⇒a complicated HJ structure near the a-Si:H/c-Si
interface region.
- The ITO/a-Si:H/c-Si interface properties are more important then the
properties of individual ITO or a-Si:H (µc-Si) layers for HJ solar cells.
- Formation of SiOx interface layer must be taken into account for the
model of HJ solar cells and for the optimization of the processing
technology.